Inductive regulation of Ly-1 alloantigen synthesis and surface expression on T cells will be defined further with the help of two well-defined mitogen-responsive T-cell lymphoma clones (derived from LBRM 33), one responding to stimulation with mitogen alone (5A4), the other requiring mitogen plus IL-1 as an inductive signal for both IL-2 as well as Ly-1 antigen expression (IA5). Information on events and sequence of Ly-1 synthesis by thymocytes deduced from 35S Met-labeling set the stage for detailed investigations of this unique Ly-1 induction system represented by two distinct clones, seemingly corresponding to two distinct normal T-cell populations. Reciprocal (inhibition of) induction, now confirmed with TP-5, IL-2 and BP, and synergistic regulation of induction, tentatively established with Il-1 and IFN-gamma, will be characterized in full-scale analysis with the use of a newly derived, more stable (towards proteases) TP5 analog, highly purified IL-2 and now crystallized BP as well as recombinant IFN-gamma. The diversity of different Ly5 forms expressed with T-\and B-cell sets will be further clarified with first emphasis on the characterization of the Ly-1+:14.8+ set predominant in MRL-1pr/1pr mice in comparison to the normally found Ly-1+:14.8- phenotype of 1pr/+ littermates and other normal mice.